The economics (i.e., cost per die) of electronic components improves significantly as feature size becomes smaller. As the size of device features becomes ever smaller, conventional lithographic processes become increasingly more difficult and expensive to use. Therefore, significant challenges are encountered in the fabrication of nanostructures, particularly structures having a feature size of less than 50 nm.
It is possible to fabricate isolated or semi-dense structures at this scale using a conventional lithographic process such as, for example, nanoimprint lithography, laser interferometry, extreme ultraviolet interference lithography, shadow mask lithography, e-beam lithography, or scanning-probe-microscopy-based lithography. However, such techniques are limited because the exposure tools are extremely expensive or extremely slow and, further, may not be amenable to formation of structures having dimensions of less than 50 nm.
The development of new processes and materials is of increasing importance in making fabrication of small-scale devices easier, less expensive, and more versatile. One example of a method of patterning that addresses some of the drawbacks of conventional lithographic techniques is block copolymer lithography, where use is made of polymer masks derived from self-assembly of block copolymers. Block copolymers are known to form nano-scale microdomains by microphase separation. When cast on a substrate and annealed, block copolymers form nano-scale periodic patterns that may be useful as an etch mask in semiconductor device fabrication. Such ordered patterns of isolated nano-sized structural units formed by the self-assembled block copolymers may potentially be used for fabricating periodic nano-scale structural units and, therefore, have promising applications in semiconductor, optical, and magnetic devices. Dimensions of the structural units so formed are typically in the range of 5 nm to 50 nm, which dimensions are extremely difficult to define using conventional lithographic techniques. The size and shape of these domains may be controlled by manipulating the molecular weight and composition of the copolymer. Additionally, the interfaces between these domains have widths on the order of 1 nm to 5 nm and may be controlled by changing the chemical composition of the blocks of the copolymers. However, the domains of the self-assembling block copolymers often have little or no etch selectivity for one another. Therefore, improving etch selectivity of the self-asssembled domains is desirable.
Buriak and Chai, “Assembly of Aligned Linear Metallic Patterns on Silicon,” Nature Nanotechnology, 2, 500-506 (Aug. 2007), discloses forming aligned metal lines by metal loading self-assembled monolayers of aligned, horizontal block copolymer cylinders using an aqueous solution of an anionic metal complex.
Cha et al., “Biometric Approaches for Fabricating High-Density Nanopatterned Arrays,” Chem Mater., 19, 839-843 (2007) discloses using the self-assembling properties of AB diblock copolymers to make polymer thin films as nanometer etch masks. A more etch-resistant film is formed by enriching the domains within the block polymer thin films with metals such as silicon.
Chai and Buriak, “Using Cylindrical Domains of Block Copolymers to Self-Assemble and Align Metallic Nanowires,” ACS Nano, 2(3), 489-501 (2008), discloses metal ion loading of self-aligned polystyrene-poly(2-vinylpyridine) block copolymers on silicon surfaces using aqueous solutions of anionic metal complexes. The basic poly(2-vinylpyridine) is protonated, rendering it cationic so that electrostatic attraction leads to a high local concentration of metal complexes within the poly(2-vinylpyridine) domain. A plasma etching process is performed to remove the polymer and form metallic nanowires.
To achieve higher-density circuits, storage devices, or displays, there is a need for less expensive fabrication techniques which are suitable for fabricating complex devices with the required enhanced density and reliable addressability of elements to meet future demands.